Insulation system for floating marine structure

ABSTRACT

Disclosed herein is a heat insulating system of a floating structure, and more particularly, a heat insulating system of a floating structure in which a heat insulating material is provided at at least one of a ballast tank, a trunk deck space, and a side passage way contacting the trunk deck to decrease a boil-off rate (BOR) occurring due to a heat transfer from an external region of a hull into an LNG storage tank.

TECHNICAL FIELD

The present invention relates to a heat insulating system of a floatingstructure, and more particularly, to a heat insulating system of afloating structure capable of decreasing a boil-off rate (BOR) bydecreasing a heat transfer through at least one of a ballast tank, atrunk deck space, and a side passage way contacting a trunk deck.

BACKGROUND ART

Generally, natural gas is transported in a gas state through a ground ormaritime gas pipe and is stored in an LNG carrier in a liquefied naturalgas (hereinafter, referred to as ‘LNG’) state to be transported toremote consumption sites.

The LNG obtained by cooling the natural gas at a very low temperature(approximately, −163° C.) has a volume decreased to approximately 1/600compared to that of the natural gas in a gas state. Therefore, the LNGis very suitable for long distance transportation through the sea.

The LNG may be transported through the sea while being carried in theLNG carrier to be loaded and unloaded at ground consumption sites or maybe transported through the sea while being carried in an LNGregasification vessel (LNG RV) to reach ground consumption sites. Afterthat, the LNG may be regasified to be loaded and unloaded in a naturalgas state. For this purpose, the LNG carrier and the LNG RV include anLNG storage tank (referred to as ‘cargo hold’) that may bear very lowtemperature LNG).

Further, a demand for floating structures such as LNG floating,production, storage and offloading (LNG FPSO) and LNG floating storageand regasification unit (LNG FSRU) is gradually increased. The floatingstructure also includes the LNG storage tank that is installed in theLNG carrier or the LNG RV.

Here, the LNG FPSO is a floating structure that directly liquefies theproduced natural gas on the sea and stores the liquefied natural gas inthe storage tank and if necessary, is used to transship the LNG storedin the storage tank to the LNG carrier.

The LNG FSRU is a floating structure that stores the LNG loaded andunloaded from the LNG carrier in the storage tank on the sea remote fromthe ground and then gasifies the LNG if necessary and supplies thegasified LNG to ground consumption sites.

The LNG storage tank is classified into an independent tank type and amembrane type depending on whether a heat insulating material forstoring LNG in a very low temperature state is directly applied to aload of freights. Here, the independent tank type storage tank isclassified into an MOSS type and an IHI-SPB type and the membrane typestorage tank is classified into a GT NO 96 type and a TGZ Mark III type.

Among the existing LNG storage tanks, the GT NO 96 type that is themembrane type has a structure in which a primary barrier and a secondarybarrier made of an invar steel (36% Ni) having a thickness of 0.5 to 0.7mm are installed on an inner surface of a hull in two layers, in whichthe primary barrier is positioned at an LNG side and the secondarybarrier is positioned at the inner surface of the hull to enclose theLNG doubly.

Further, a space between the primary barrier and the secondary barrieris provided with a primary heat insulating wall and a space between thesecondary barrier and an internal hull is provided with a secondary heatinsulating wall, in which the primary heat insulating wall and thesecondary heat insulating wall minimize that heat outside the LNGstorage tank is transferred to the LNG.

Meanwhile, since the LNG stored in the LNG storage tank is stored atapproximately −163° C. that is a gasification temperature at a normalpressure, if heat is transferred to the LNG, the LNG is gasified andthus boil off gas (hereinafter, referred to as ‘BOG’) occurs.

Further, in the case of the membrane type LNG storage tank, if the coldLNG storage tanks are continuously installed, a temperature of steelbetween the cold LNG storage tanks is suddenly decreased, and thereforea brittle fracture may occur.

To prevent the brittle fracture, a space called the cofferdam isinstalled between the LNG storage tanks and thus the LNG storage tank isprevented from being damaged due to the low temperature LNG.

However, even though the cofferdam is installed, the temperature ofsteel of a cofferdam bulk head member contacting the LNG cargo hold maybe dropped to −60° C. or less due to the very low temperature LNG. Ifthe general steel is exposed to −60° C., the cofferdam is damaged due tolow temperature brittleness.

As a plan to overcome the above problem, the cofferdam may be made of avery low temperature material such as stainless steel and aluminum.However, if the very low temperature material is used, ship prices maysuddenly rise.

Therefore, when the cofferdam and the LNG storage tank are installed,the temperature of the cofferdam is controlled to be 5° C. and thecofferdam bulk head is made of relatively cheap steel that can bear theroom temperature.

In the case of the existing LNG carrier, when the temperature of thecofferdam is equal to or less than 5° C., a heating system is operatedand thus the cofferdam is always maintained at 5° C. or more. For thispurpose, the existing LNG carrier includes a glycol heating system or anelectrical heating system.

Therefore, the existing LNG carrier is designed/sailed so that thetemperature of the cofferdam may be equal to or more than at least 5° C.all the time and a BOR is also generated under the temperaturecondition.

DISCLOSURE Technical Problem

In the current LNG carrier market, the LNG carrier is sensitive tonumerical values of the BOR at a contract stage. As an actual example,typically, the BOR of 0.15% is a contract condition, but in recentyears, the BOR of 0.125%, 0.10%, 0.095%, or the like as the contractcondition may be suggested.

However, in the current membrane type tank, the heat insulating wall isinstalled in the cargo hold Since the heat insulating wall of the LNGcargo hold needs to bear and transfer the load applied from the LNGfreight to the cargo hold while having heat insulating performance, theexisting heat insulating wall of the LNG cargo hold is changed toincrease the heat insulating performance, a lot of research and designchanges may be involved and costs may rise.

In fact, even though there is the insulating wall of the LNG cargo holdsatisfying the BOR of about 0.13%, if the BOR requirement of aship-owner is 0.125%, to decrease the BOR as much as about 4%, a lot ofresearch, time, and costs are involved.

Further, even though there is the insulating wall of the LNG cargo holdsecuring the BOR of 0.103%, if the ship-owner proposes the BOR of 0.10%,shipyards may not apply the LNG cargo hold and therefore may not acceptan order for the LNG carrier. Today's LNG carrier market is in asituation that a shipyard achieving even 1% decrease in the BOR maydominate an order competition for the LNG carrier over other shipyards.

Meanwhile, the existing technology development for decreasing the BORfocuses on improvement in the performance of the insulating wall of theLNG cargo hold. Since today's market demands even 1% decrease in theBOR, a method mostly discussed at present is to increase a thickness ofthe LNG cargo hold.

However, a volume of the cargo hold that may store LNG is decreased inproportion to the increase in the thickness of the LNG cargo hold. Onthe other hand, to prevent the volume of the cargo hold from decreasing,a size of a ship is increased.

Further, if the thickness of the cargo hold is increased, the cargo holdis weaker structurally. Therefore, researches for reinforcing the cargohold have to be conducted.

An aspect of the present invention provides a heat insulating system ofa floating structure, and more particularly, a heat insulating system ofa floating structure capable of decreasing a boil-off rate (BOR) bydecreasing a heat transfer through at least one of a ballast tank, atrunk deck space, and a side passage way contacting a trunk deck.

Technical Solution

According to an embodiment of the present invention, there is provided aheat insulating system of a floating structure, in which a heatinsulating material is provided at at least one of a ballast tank, atrunk deck space, and a side passage way contacting the trunk deck todecrease a boil-off rate (BOR) occurring due to a heat transfer from anexternal region of a hull into an LNG storage tank.

A temperature of an internal hull contacting at least one of the ballasttank, the trunk deck space, and the side passage way contacting thetrunk deck may be controlled to be −55 to 30° C.

The temperature of an internal hull contacting the ballast tank may becontrolled to be 0 to 20° C. and the internal hull may be made of steelgrade A defined in IGC.

The temperature of the internal hull contacting at least any one of thetrunk deck space and the side passage way contacting the trunk deck maybe controlled to be 0 to 30° C. and the internal hull may be made of thesteel grade A defined in the IGC.

The temperature of the internal hull contacting at least any one of thetrunk deck space and the side passage way contacting the trunk deck maybe controlled to be −30° C. or less and the internal hull may be made oflow temperature steel LT.

The heat insulating material may be provided on an inside wall of anexternal hull forming the ballast tank.

The heat insulating material may be provided in a region to which theside passage way close to the trunk deck space and the ballast tank arecontacted.

The heat insulating material may include at least one of the heatinsulating wall heat-insulating LNG stored in the LNG storage tank, apanel type heat insulating material, a foam type heat insulatingmaterial, a vacuum type heat insulating material, a particle type heatinsulating material, and a non-woven fabrics type heat insulatingmaterial.

The floating structure may further include a heater heating the internalhull.

The heater may use at least of a glycol heating coil, an electricheating coil, and a fluid pipe to heat the internal hull.

The floating structure may be any one selected from an LNG FPSO, an LNGFSRU, an LNG carrier, and an LNG RV.

According to another embodiment of the present invention, there isprovided a heat insulating system of a floating structure including: acofferdam provided between a plurality of LNG storage tanks installed inat least one row in a length direction of a hull; and a heat insulatingmaterial provided in a ballast tank, a trunk deck space, and a sidepassage way contacting a trunk deck, in which the cofferdam iscontrolled to be a temperature of 0° C. or less to decrease a heattransfer from the cofferdam into the plurality of LNG storage tanks andcontrols a temperature of at least one of the ballast tank, the trunkdeck space, and the side passage way contacting the trunk deck to belower than a temperature of an external hull to decrease a boil-off rate(BOR) generated due to the heat transfer from at least one of theballast tank, the trunk deck space, and the side passage way contactingthe trunk deck into the plurality of LNG storage tanks.

The cofferdam may include: a pair of bulk heads spaced apart from eachother between the plurality of LNG storage tanks; and a space portionprovided by the pair of bulk heads and an inner wall of the hull, andthe cofferdam controls the pair of bulk heads to be temperature below 0°C.

According to another embodiment of the present invention, there isprovided a heat insulating system of a floating structure, in which atemperature of an internal hull contacting at least one of a ballasttank, a trunk deck space, and a side passage way contacting a trunk deckmay be controlled to be −55 to 30° C.

Advantageous Effects

According to the embodiments of the present invention, it is possible tocontrol the temperature of the cofferdam to be temperature below 0° C.and provide the heat insulating material at at least one of the ballasttank, the trunk deck space, and the side passage way contacting thetrunk deck, thereby decreasing the heat transfer from the cofferdam intothe plurality of LNG storage tanks and decrease the heat transfer fromat least one of the ballast tank, the trunk deck space, and the sidepassage way contacting the trunk deck into the plurality of LNG storagetanks, thereby decreasing the boil-off rate (BOR) generated due to theheat transfer.

That is, the embodiments of the present invention do not decrease theBOR by deforming the complex and expensive LNG cargo hold butfundamentally decrease the heat invasion into the LNG cargo hold bylowering temperature around the LNG cargo hold, thereby decreasing theBOR while maintaining the transportation efficiency of the LNG freight.

DESCRIPTION OF DRAWINGS

FIG. 1 is a side view schematically illustrating a state in which acofferdam is installed in a floating structure according to a firstembodiment of the present invention.

FIG. 2 is a cross-sectional view taken along the line II-II of FIG. 1.

FIG. 3 is a cross-sectional view taken along the line III-III of FIG. 1.

FIG. 4 is a plan cross-sectional view illustrating a state in which thecofferdam is provided between LNG storage tanks disposed in two rows inthe floating structure illustrated in FIG. 1.

FIG. 5 is a cross-sectional view taken along the line IV-IV of FIG. 4.

FIG. 6 is a table showing a steel grade defined in IGC.

FIG. 7 is a table showing calculated results of a BOR generated bycontrolling a temperature of the cofferdam in the first embodiment ofthe present invention.

FIG. 8 is a diagram schematically illustrating a state in which a heateris provided in the floating structure according to the first embodimentof the present invention.

FIG. 9 is a diagram schematically illustrating a state in which a heatinsulating material is provided in a cofferdam in a heat insulatingsystem of a floating structure according to a second embodiment of thepresent invention.

FIG. 10 is a perspective view schematically illustrating a state inwhich the heat insulating material is provided in region “A” of FIG. 9.

FIG. 11 is a perspective view schematically illustrating a state inwhich the heat insulating material is provided in region “B” of FIG. 9.

FIGS. 12A and 12B are diagrams schematically illustrating a heatinsulating material damage prevention member provided to prevent theheat insulating material from being damaged in region “C”.

FIG. 13 is a table illustrating calculated results of a BOR generated bycontrolling the temperature of the cofferdam using the heat insulatingmaterial illustrated in FIG. 9.

FIG. 14 is a diagram schematically illustrating a state in which a bulkhead of a cofferdam in a floating structure according to a thirdembodiment of the present invention is not extended to an external hullbut is connected only to an internal hull.

FIG. 15 is a modified example of FIG. 14 in which instead of the bulkhead illustrated in FIG. 14, the cofferdam is provided and the heatinsulating material is provided in the cofferdam.

FIG. 16 is a table illustrating calculated results of a BOR generated bymanufacturing the bulk head illustrated in FIG. 13 using a very lowtemperature material and controlling the temperature of the cofferdam.

FIG. 17 is a diagram schematically illustrating a gas supplier in afloating structure according to a fourth embodiment of the presentinvention.

FIG. 18 is a table illustrating calculated results of a BOR generated bycontrolling a temperature of a cofferdam illustrated in FIG. 17.

FIG. 19 is a diagram schematically illustrating controlling thetemperature of the cofferdam depending on a change in pressure of an LNGstorage tank in a floating structure according to a fifth embodiment ofthe present invention.

FIG. 20 is a diagram schematically illustrating a state in which a heatinsulating material is provided in a trunk deck space and a side passageway in a heat insulating system of a floating structure according to asixth embodiment of the present invention.

FIG. 21 is a table showing calculated results of a BOR generated bycontrolling a temperature of an internal hull contacting the trunk deckspace and the side passage way illustrated in FIG. 20.

FIG. 22 is a diagram schematically illustrating a state in which a heatinsulating material is provided in a ballast tank in a heat insulatingsystem of a floating structure according to a seventh embodiment of thepresent invention.

FIG. 23 is a table illustrating calculated results of a BOR generated bycontrolling a temperature of an internal hull contacting the ballasttank.

DETAILED DESCRIPTION OF MAIN ELEMENTS

-   -   1, 200, 300, 400: Floating structure    -   100, 500, 600: Heat insulating system of floating structure    -   10: Cofferdam 30: Heater    -   120: Heat insulating material 220: Strength member    -   320: Gas supplier

EMBODIMENTS

In order to sufficiently understand the present invention, operationaladvantages of the present invention, and objects accomplished byembodiments of the present invention, the accompanying drawings showingembodiments of the present invention and contents described in theaccompanying drawings should be referred.

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. Like referencenumerals proposed in each drawing denote like components.

In the present specification, a floating structure includes a ship andvarious structures that are used while floating on the sea, including astorage tank for storing LNG and may include an LNG floating,production, storage and offloading (LNG FPSO), an LNG floating storageand regasification unit (LNG FSRU), an LNG carrier, and an LNGregasification vessel (LNG RV).

FIG. 1 is a side view schematically illustrating a state in which acofferdam is installed in a floating structure according to a firstembodiment of the present invention, FIG. 2 is a cross-sectional viewtaken along the line II-II of FIG. 1, FIG. 3 is a cross-sectional viewtaken along the line III-III of FIG. 1, FIG. 4 is a plan cross-sectionalview illustrating a state in which the cofferdam is provided between LNGstorage tanks disposed in two rows in the floating structure illustratedin FIG. 1, FIG. 5 is a cross-sectional view taken along the line IV-IVof FIG. 4, FIG. 6 is a table showing a steel grade defined in IGC, FIG.7 is a table showing calculated results of a BOR generated bycontrolling a temperature of the cofferdam in the first embodiment ofthe present invention, and FIG. 8 is a diagram schematicallyillustrating a state in which a heater is provided in the floatingstructure according to the first embodiment of the present invention.

According to the present embodiment, a cofferdam 10 is controlled to betemperature below 0° C. to decrease a boil-off rate (BOR) generated bytransferring heat from the cofferdam 10 into a plurality of LNG storagetanks T.

As illustrated in the drawings, a floating structure 1 according to thepresent embodiment includes the cofferdam 10 provided between theplurality of LNG storage tanks T installed in at least one row in alength direction of a hull and controlled to be temperature below 0° C.

The cofferdam 10 is provided in the plurality of LNG storage tanks Tinstalled in at least one row in the length direction of the hull and asillustrated in FIGS. 1 to 3, may be provided between the plurality ofLNG storage tanks T installed in multi rows in the length direction ofthe hull or as illustrated in FIGS. 4 and 5, provided between theplurality of LNG storage tanks T installed in two rows in a widthdirection and the length direction of the hull.

Unlike the related art, according to the present embodiment, thecofferdam 10 is controlled to be temperature below 0° C. to decrease theBOR.

In detail, the related art always maintains a temperature of thecofferdam at 5° C. or more. The reason is that when the temperature ofthe cofferdam is controlled to be lower than 5° C., a temperature of thebulk head 11 of the cofferdam using steel grade A defined in IGC islowered below 0° C. and the bulk head 11 may suffer from brittlefracture.

When the temperature of the cofferdam is maintained at 5° C. or more asdescribed above, the BOR is generated by heat transfer due to atemperature difference between the cofferdam and the LNG stored in theLNG storage tank T. For example, for an actual ship built in a shipyard,as illustrated in a table of FIG. 6, a BOR of 0.1282 is calculated.

However, according to the present embodiment, when the temperature ofthe cofferdam 10 is controlled to be temperature below 0° C., thetemperature difference between the LNG and the cofferdam 10 is decreasedand thus the heat transfer between the LNG and the cofferdam 10 is moredecreased than the related art, which leads to the decrease in the BOR.

According to the embodiment of the present invention, when the bulk headof the cofferdam is made of a material that may bear a temperature from−30° C. to 0° C., the temperature of the cofferdam may be changed in arange from −30° C. to 70° C. and when the bulk head of the cofferdam ismade of low temperature steel LT that may bear up to a temperature of−55° C., in detail, a temperature from −31° C. to −55° C., thetemperature of the cofferdam may be changed in a range from −55° C. to70° C.

In detail, as illustrated in a table of FIG. 7, when the temperature ofthe bulk head 11 of the cofferdam 10 is controlled to be −25° C., thetemperature of the cofferdam 10 may be maintained at −20.8° C. In thiscase, the BOR becomes 0.1236 and it may be appreciated that the value is3.5% smaller than the BOR of the related art.

Further, as illustrated in the table of FIG. 7, when the temperature ofthe bulk head 11 of the cofferdam 10 is controlled to be −50° C., thetemperature of the cofferdam 10 may be maintained at −46.5° C. In thiscase, the BOR becomes 0.1192 and it may be appreciated that the value is7.0% smaller than the BOR of the related art. For reference, theforegoing value of the BOR is a result of numerical analysis.

However, when the temperature of the cofferdam 10 is maintained at thetemperature below 0° C., the bulk head 11 needs to be made of thematerials defined in the IGC or the low temperature steel LT andtherefore costs may be expected to be increased. However, the increasein costs is smaller than a profit generated at the time of the decreasein the BOR, and therefore the BOR may be effectively decreased withrelatively lower cost.

Further, a loss of LNG evaporated as the BOG due to the decrease in theBOR may be prevented and therefore the foregoing increase in costs maybe sufficiently offset.

Describing in detail the cofferdam 10, according to the presentembodiment, as illustrated in FIG. 1, the cofferdam includes a pair ofbulk heads 11 disposed between the plurality of LNG storage tanks Twhile being spaced apart from each other and a space portion 12 providedby the pair of bulk heads 11 and an internal hull IH and may control thepair of bulk heads 11 to be temperature below 0° C. to control thetemperature of the cofferdam 10 to be temperature below 0° C.

According to the present embodiment, the temperature of the cofferdam 10may be controlled to be temperature below 0° C. by, for example,controlling a set temperature at which a heating system of the cofferdam10 is operated, additionally installing a heat insulating material 120(see FIG. 9) in the cofferdam 10, or injecting cooled gas in thecofferdam 10.

In detail, upon designing an LNG carrier, external air temperaturebecomes −18° C. depending on USCG conditions and the LNG carrier needsto be designed without any problem even when sea temperature is 0° C.Under the external temperature condition, when the cofferdam 10 is notheated, the cofferdam 10 is dropped up to −60° C. by cold heat of theLNG stored in the LNG storage tank T.

Therefore, the related art heats the cofferdam 10 to control thetemperature of the space portion 12 of the cofferdam 10 to be 5° C. andthe temperature of the bulk head 11 to be 0° C. or more all the time.

However, according to the present embodiment, unlike the existing LNGcarrier, in the temperature below 0° C. proposed in the presentembodiment, a heater may be operated to control the temperature of thecofferdam 10 to be a specific sub-zero temperature.

Further, the heat insulating material 120 (see FIG. 9) may be installedinside the cofferdam 10 to control the temperature of the cofferdam 10to be temperature below 0° C. and the heat insulating material 120 willbe described in detail in a second embodiment to be described below.

According to the present embodiment, the foregoing method forcontrolling the temperature of the cofferdam 10 to be temperature below0° C. may also be independently used and may also be used like othermethods, and therefore the scope of the present invention is not limitedto applying any one method.

The temperature of the bulk head 11 of the cofferdam 10 is controlled tobe temperature below 0° C. and therefore the bulk head 11 may be made ofB, D, E, AH, DH, and EH that are a steel grade defined in the IGC.

In particular, when the temperature of the bulk head 11 of the cofferdam10 is controlled to be a temperature from −30° C. to −20° C., the bulkhead 11 may be made of E or EH that is the steel grade defined in theIGC and when the temperature of the bulk head 11 is controlled to betemperature from −60° C. to −30° C., the bulk head 11 may be made of thelow temperature steel LT.

According to the present embodiment, when the bulk head 11 is made ofthe low temperature steel, the low temperature steel may be made of oneof low temperature carbon steel, low temperature alloy steel, nickelsteel, aluminum steel, and austenite-based stainless steel or at leastone combination thereof.

Further, as illustrated in FIGS. 1 and 3, when the cofferdam 10 isdisposed in one row in the width direction of the hull, in the spaceportion 12, the pair of bulk heads 11 spaced apart from each other inthe length direction of the hull may form a front wall 7 a and a rearwall 9 a and the internal hull IH may form left and right side walls, aceiling part, and a bottom part.

Further, according to the present embodiment, as illustrated in FIG. 4,the cofferdam 10 includes a lateral cofferdam 10 a laterallypartitioning an internal space of the LNG storage tank T and alongitudinal cofferdam 10 b longitudinally partitioning it.

In this case, in the space portion 12 of the cofferdam 10, in the caseof the lateral cofferdam 10 a, as illustrated in FIG. 4, the pair ofbulk heads 11 spaced apart from each other in the length direction ofthe hull may each form a front wall and a rear wall of the space portion12, the right internal hull IH may form a right wall 3 a, a leftpartition wall may form a left wall 5 a, and the internal hull IH mayform a ceiling wall and a floor wall.

Further, in the case of the longitudinal cofferdam 10 b, as illustratedin FIG. 4, the pair of bulk heads 11 spaced apart from each other in thewidth direction of the hull may each form the right wall and the leftwall of the space portion 12, a wall contacting the bulk head 11 of thelongitudinal cofferdam 10 b and bulk head 11 of the lateral cofferdam 10a may form a front wall 7 a and a rear wall 7 b, and the internal hullIH may form the ceiling wall and the floor wall.

According to the present embodiment, the space portion 12 may beprovided with the heat insulating material 120 according to the secondembodiment of the present invention to be described below, and the heatinsulating material 120 will be described in detail in the secondembodiment of the present invention.

A gas supplier supplies gas into the cofferdam 10 to serve to preventthe cofferdam 10 from being damaged due to frost covered on thecofferdam 10, a change in humidity, or the like.

According to the present embodiment, the gas supplier may be configuredto be the same as a gas supplier 300 (see FIG. 17) according to a fourthembodiment of the present invention to be described below and includes asupply pipe branched from a gas supply line to supply gas suppliedthrough the gas supply line into the cofferdam 10, a discharge pipeprovided in the cofferdam 10 to discharge gas filled in the cofferdam 10to the outside of the cofferdam 10, and valves provided in the supplypipe and the discharge pipe.

The supply pipe of the gas supplier may be provided in the numbercorresponding to the number of cofferdams 10 and a lower end portion ofthe supply pipe may be disposed to be close to the bottom of thecofferdam 10.

The discharge pipe of the gas supplier may be provided in the numbercorresponding to the number of cofferdams to discharge the gas filled inthe cofferdam 10 and the discharge pipes may also discharge gas by beingconnected to each other.

The valve for the gas supplier 20 may be a proportional control valvethat is opened and closed by an electrical signal.

According to the present embodiment, the gas supplied to the gas supplyline includes dry air, inert gas, or N2 gas and the gas may be suppliedfrom the existing dry air/inert gas generator that is already installedin the LNG carrier.

Meanwhile, according to the present embodiment, the floating structuremay include a heater 30 controlling the temperature of the cofferdam 10dropped to a first sub-zero temperature to be a second sub-zerotemperature higher than the first sub-zero temperature.

According to the present embodiment, as illustrated in FIG. 8, theheater 30 may also heat the bulk head 11 by installing a glycol heatingcoil 31 in the cofferdam 10 and supplying heated glycol to the glycolheating coil 31 and may also heat the bulk head 11 by installing anelectrical coil in the cofferdam 10.

Further, a coil through which waste heat of exhaust gas or hightemperature liquid or steam may be circulated may also be provided inthe cofferdam 10 to heat the bulk head 11.

According to the present embodiment, when the glycol is used as ananti-freezing solution, 45% of glycol water having a freezing point of−30° C. may be used.

A method for heating glycol supplied to a cofferdam 10 will be brieflydescribed with reference to FIG. 8.

The glycol circulated by a glycol circulation pump is heated with thehigh temperature steam supplied from a boiler, or the like in a glycolheater GH prior to being supplied to the cofferdam 10 and the heatedglycol is supplied to the glycol heating coil 31 provided in thecofferdam 10 to heat the bulk head 11 and then be circulated.

According to the present embodiment, the cofferdam 10 may be providedwith a temperature sensor TS that may measure the temperature of theinside of the cofferdam 10 and when the temperature of the inside of thecofferdam 10 is lower than the set value, the heated glycol may besupplied to the glycol heating coil 31 attached to the bulk head 11 toincrease or maintain the temperature of the bulk head 11 and the spaceportion 12.

Meanwhile, since the freezing point of the anti-freezing solution may bedropped to −50° C. or less when the temperature of the bulk head 11 iscontrolled to be −50° C. or less, as the anti-freezing solution, 65% ofglycol water or methyl alcohol may be used. The contents described inthe first embodiment of the present invention may be applied to otherembodiments to be described below as it is.

FIG. 9 is a diagram schematically illustrating a state in which a heatinsulating material is provided in a cofferdam in a heat insulatingsystem of a floating structure according to a second embodiment of thepresent invention, FIG. 10 is a perspective view schematicallyillustrating a state in which the heat insulating material is providedin region “A” of FIG. 9, FIG. 11 is a perspective view schematicallyillustrating a state in which the heat insulating material is providedin region “B” of FIG. 9, FIGS. 12A and 12B are diagrams illustrating amodified example of the heat insulating material provided in region “C”,and FIG. 13 is a table illustrating calculated results of a BORgenerated by controlling the temperature of the cofferdam using the heatinsulating material illustrated in FIG. 9.

A heat insulating system 100 of the floating structure according to thepresent embodiment includes the heat insulating material 120 provided inthe cofferdam 10 for controlling the temperature of the cofferdam 10 tobe temperature below 0° C., independent of spatial environment such as apolar region and a tropical region or temporal environment such asseason and week/day and night.

As illustrated in FIG. 9, the heat insulating material 120 is providedin the cofferdam 10 to serve to prevent heat from being invaded into thecofferdam 10 when the floating structure is sailed in a high temperatureregion or during the summer to drop the temperature of the cofferdam 10to the desired temperature even when the outside temperature is high.

In detail, when the outside temperature is high, for example, when theoutside air temperature proposed in an IGC code is 45° C. and the seatemperature is 25° C., as illustrated in a table of FIG. 13, without theheat insulating material 120, the temperature of the cofferdam 10 may bedropped only to −15.39° C. and thus the decrease in the BOR may belimited.

However, according to the present embodiment, if the cofferdam 10 isprovided with the heat insulating material 120, the temperature of thecofferdam 10 is lowered to the desired temperature, for example, −25° C.and −50° C. even under the foregoing temperature conditions and thus thesufficient BOR decrease effect may be obtained.

Describing in detail the heat insulating material 120, according to thepresent embodiment, the heat insulating material 120 may use a heatinsulating wall that is a type different from the foregoing type of heatinsulating wall in consideration of operation convenience and cost,etc., as well as the heat insulating wall used to heat-insulate the LNGstored in the LNG storage tank T.

That is, according to the present embodiment, the heat insulatingmaterial 120 may include at least one of a panel type heat insulatingmaterial, a foam type heat insulating material, a vacuum heat insulationor particle type heat insulating material, and a non-woven fabrics typeheat insulating material that are types different from the foregoingheat insulating wall.

According to the present embodiment, the heat insulating material 120may be applied without a limitation of kind and shape. Only any one ofthe foregoing three types of heat insulating materials may be used inconsideration of the operation environment and cost, etc., and at leasttwo heat insulating materials may also be selectively used. In addition,the heat insulating wall heat-insulating the LNG stored in the LNGstorage tank T may also be used. Here, the heat insulating wall refersto a heat insulating wall of a sealing and heat insulating unit SI.

The panel type heat insulating material includes styrofoam, in which thestyrofoam may be coupled to the cofferdam 10 by an attachment schemeusing a low temperature adhesive, a bolt, etc.

The foam type heat insulating material includes polyurethane foam, inwhich the polyurethane foam may be injected to the cofferdam 10 by afoaming scheme to be coupled to the cofferdam 10.

The non-woven fabrics type heat insulating material may also be made ofa polyester fiber material or a synthetic resin and may be coupled tothe cofferdam 10 by the attachment scheme using the low temperatureadhesive, the bolt, etc.

According to the present embodiment, the kind and installation method ofthe heat insulating material 120 are not limited.

According to the present embodiment, as illustrated in FIG. 9, the heatinsulating material 120 may be provided in the space portion 12 of thecofferdam 10 of a region other than the pair of bulk heads 11.

In detail, as illustrated in FIG. 10, in the case of the lateralcofferdam 10 a, the heat insulating materials 120 may be each providedat a right wall part, a left wall part, a ceiling part, and a bottompart of the space portion 12 of the cofferdam 10. Further, the heatinsulating material 120 provided at the ceiling part and the bottom partmay not be provided inside the space portion 12 but provided outside thespace portion 12.

If the heat insulating material 120 is provided in the cofferdam 10 ofthe region other than the pair of bulk heads 11, heat outside the regionthat does not contact the pair of bulk heads 11 may be prevented frombeing invaded into the cofferdam 10 and the cold heat of the LNG storedin the LNG storage tank T may be transferred to the space portion 12through the pair of bulk heads 11, such that the temperature of thecofferdam 10 may be lowered to the desired temperature even when thetemperature of the outside of the hull is high.

Further, according to the embodiment of the present invention, the heatinsulating materials 120 may be provided in a foremost bulk head 11 of abow of the lateral cofferdam 10 a disposed at a foremost side of the bowand a rearmost bulk head of a stern of the lateral copper dam 10 adisposed at a rearmost side of the stern, respectively, among theplurality of lateral cofferdams 10 a.

In detail, FIG. 11 illustrates that the heat insulating material 120 isprovided in the bulk head 11 of the foremost side of the bow, in whichthe foremost side of the bow and the rearmost side of the stern haveenvironment different from a region between the bow and the stern.

That is, the regions of the foremost side of the bow and the rearmostside of the stern contact the LNG storage tank T only in one directionand contact the inner wall of the hull, such that it is more difficultto lower the temperature of the cofferdam 10 to the desired temperaturethan lowering the temperature of the cofferdam 10 disposed in the regionbetween the bow and the stern.

However, according to the present embodiment, if the foremost bulk head11 of the bow and the rearmost bulk head 11 of the stern are providedwith the heat insulating material 120, heat may be prevented from beinginvaded from the outside and therefore the temperature of the cofferdam10 may be lowered to the desired temperature.

Meanwhile, when the inside of the cofferdam 10 is provided with the heatinsulating material 120, the heat insulating material 120 provided atthe bottom part of the cofferdam 10 may be damaged due to a crew Thatis, when a worker enters the cofferdam 10, he/she stands on the bottompart of the cofferdam 10 with his/her feet. In this case, the heatinsulating material 120 may be damaged.

Therefore, according to the embodiment of the present invention, toprevent the heat insulating material 120 from being damaged as describedabove, as illustrated in FIG. 12, a heat insulating material damageprevention member may be provided.

According to the embodiment of the present invention, as illustrated inFIG. 12A, a heat insulating material damage prevention member 130 a maybe provided in a grid form and disposed on the heat insulating material120 to prevent a load from being concentrated on a specific part of theheat insulating material 120, thereby preventing the heat insulatingmaterial 120 from being damaged.

Further, a heat insulating material damage prevention member 130 b maybe a separate path provided at the bottom part of the cofferdam 10 tomove a crew to a desired place. A main area to which a crew isapproached is an edge of the bottom part, and therefore as illustratedin FIG. 12B, the heat insulating material damage prevention member 130 bmay be provided only at the edge of the bottom part of the cofferdam 10,having a slight width.

FIG. 13 illustrates the decrease effect of the BOR by the installationof the heat insulating material and the temperature control of thecofferdam.

As before, if the cofferdam is controlled to be 5° C., the BOR becomesabout 0.1282. Here, to control the temperature of the cofferdam, evenwhen the control temperature of the glycol heating system is controlled,that is, when the glycol heating is not performed, the cofferdam may bedropped only to −10.87° C. even at the time of the lowest temperature.

Therefore, even though the bulk head 11 of the cofferdam is made of thesteel grade E that may bear up to −25° C., the temperature of thecofferdam is dropped only to −15.39° C. and therefore the BOR isdecreased only by about 2.2%.

However, by applying the present embodiment, if the heat insulatingmaterial 120 is installed to drop the temperature of the cofferdam 10 upto −26.4° C. and increase the temperature of the cofferdam 10 up to−20.8° C. by the glycol heating, the BOR may be decreased by about 3.5.

Further, the contents of the first embodiment of the present inventionas described above may be applied to the present embodiment as it is.

FIG. 14 is a diagram schematically illustrating a state in which a bulkhead of a cofferdam in a floating structure according to a thirdembodiment of the present invention is not extended to an external hullbut is connected only to an internal hull, FIG. 15 is a modified exampleof FIG. 14 in which instead of the bulk head illustrated in FIG. 14, thecofferdam is provided and the heat insulating material is provided inthe cofferdam, and FIG. 16 is a table illustrating calculated results ofa BOR generated by manufacturing the bulk head illustrated in FIG. 13using a very low temperature material and controlling the temperature ofthe cofferdam.

A heat insulating system 200 of the floating structure according to thepresent embodiment includes a bulk head 210 that is provided between theplurality of LNG storage tanks T to dispose the plurality of LNG storagetanks T in multi rows in at least one of a length direction and a widthdirection of the hull and is not extended up to an external hull EH butis connected only to the internal hull IH, a strength member 220 thatconnects between the internal hull IH and the external hull EH toreinforce the internal hull IH and the external hull EH and is notcontinued from the bulk head 210, the heat insulating material 120provided at the foremost side of the bow and the rearmost side of thestern, the gas supplier 20 that supplies gas to the space portion 12provided by the bulk heads 210 at the foremost side of the bow and therearmost side of the stern to prevent the space from being damaged dueto the change in humidity, and the heater 30 that heats the bulk heads210 provided at the foremost side of the bow and the rearmost side ofthe stern.

As illustrated in FIG. 14, the bulk head 210 may dispose the LNG storagetank T in the multi row in the length direction of the hull and in themulti row in the width direction of the hull.

Further, according to the present embodiment, the region to which thebulk head 210 and the LNG storage tank T are contacted is not providedwith the sealing and heat insulating unit SI and the heat insulatingmaterial 120 and therefore the temperature of the bulk head 210 may bedropped to a very low temperature of −140° C.

Therefore, according to the present embodiment, the bulk head 210 may bemade of the very low temperature materials including stainless steel oraluminum and an end portion of a sealing wall of the sealing and heatinsulating unit SI sealing and heat-insulating the LNG storage tank Tmay be directly welded to the bulk head 210.

Further, according to the present embodiment, a pair of bulk heads 210may be spaced apart from each other at the foremost side of the bow andthe rearmost side of the stern to provide the space portions 12 at theforemost side of the bow and the rearmost side of the stern. The bulkhead 210 of the space portion 12 may be provided with the heatinsulating material 120 and the heater 30 and the gas supplier maysupply gas to the space portion 12 to prevent the bulk head 210 frombeing damaged.

Meanwhile, unlike the related art, according to the present embodiment,as illustrated in FIG. 14, the bulk head 210 is not extended up to theexternal hull EH. The reason is that if the bulk head 210 is connectedup to the external hull EH, heat is transferred from the outside throughthe bulk head 210 and thus the BOR may also be increased and theexternal hull EH contacts the bulk head 210 and therefore the brittlefracture may occur due to the cold heat transferred from the bulk head210.

As illustrated in FIG. 14, the strength member 220 connects between theinternal hull IH and the external hull EH at an intermediate position ofthe LNG storage tank T to serve to structurally reinforce the hull.

As illustrated in FIG. 14, according to the present embodiment, thestrength member 220 is provided not to be continued from the bulk head210 and therefore the cold heat transferred through the bulk head 210may be offset by the sealing and heat insulating units SI provided atboth end portions of the bulk head 210 and the bulk head 210 does notdirectly contact the external hull EH and therefore the heat transferfrom the outside may also be decreased.

According to the present embodiment, the strength member 220 may beprovided even at any position as long as it is disposed not to becontinued from the bulk head 210 and the number of strength members 220is not limited.

Further, the strength member 220 is not exposed to very low temperatureand therefore may also be made of steel that is the steel grade A.

As the heat insulating material 120, the heat insulating material 120according to the foregoing second embodiment may be applied as it is.However, there is a difference in that the heat insulating material 120is not provided between the LNG storage tanks T but is provided at theforemost side of the bow and the rearmost side of the stern, in theinstallation position.

The gas supplier and the heater 30 may be applied like the foregoingfirst embodiment. However, there is a difference from the foregoingfirst embodiment in that they are applied to the space portions 12provided at the foremost side of the bow and the rearmost side of thestern.

According to the present embodiment, not the cofferdam 10, the bulk head210 is provided between the LNG storage tanks T and thus it is difficultto directly control the temperature of the bulk head 210 disposedbetween the LNG storage tanks T, such that the foregoing bulk head 210is controlled to be about −130° C. due to the direct contact with LNG.

However, the temperature of the bulk heads 210 disposed at the foremostside of the bow and the rearmost side of the stern may be freelycontrolled by the heater 30 and even in the case of the bulk head 210disposed between the LNG storage tanks T, the temperature of the bulkhead 210 may be controlled by controlling the heat insulating wall ofthe sealing and heat insulating unit SI or heating both ends of the bulkhead 210 with an electrical coil.

Further, as illustrated in FIG. 15, the bulk head 210 may also beprovided in at least two, the bulk heads 210 provided in at least twomay also be spaced apart from each other, and the present embodiment mayalso be applied to a double hull structure configured of the internalhull IH and the external hull EH.

Meanwhile, according to the present embodiment, as illustrated in FIGS.14 and 15, the region to which the bulk head 11 and the LNG storage tankT are contacted may not be provided with the sealing and heat insulatingunit SI. In this case, if the bulk head 11 is made of the very lowtemperature material and the temperature of the cofferdam 10 iscontrolled to be temperature below 0° C., the BOR may be obtained asillustrated in FIG. 16.

In detail, as illustrated in FIG. 14, if the region to which the bulkhead 11 and the LNG storage tank T are contacted is not provided withthe sealing and heat insulating unit SI, the cold heat of the LNG storedin the LNG storage tank T is transferred to the cofferdam 10 well andthus the temperature of the cofferdam 10 may be dropped to −125° C. asillustrated in FIG. 16. In this case, it may be appreciated that the BORis 0.1061 decreased by 17.2%, compared to the case of controlling thetemperature of the cofferdam 10 to be 5° C.

In this case, the bulk head 11 of the cofferdam 10 may be controlled tobe a temperature from −163 to −50° C., the bulk head 11 may be made ofthe very low temperature material including stainless steel or aluminum,not a general material, and the sealing wall of the sealing and heatinsulating unit SI contacting the bulk head 11 may be coupled to thebulk head 11 by a welding scheme.

FIG. 17 is a diagram schematically illustrating a gas supplier in afloating structure according to a fourth embodiment of the presentinvention and FIG. 18 is a table illustrating calculated results of aBOR generated by controlling a temperature of a cofferdam illustrated inFIG. 17.

A floating structure 300 according to an embodiment of the presentinvention includes the cofferdam 10 provided between the plurality ofLNG storage tanks T to dispose the plurality of LNG storage tanks T inmulti rows in at least any one direction of the length direction and thewidth direction of the hull and controlled to be temperature below 0°C., a gas supplier 320 supplying gas to the cofferdam 10, the heater 30provided in the cofferdam 10 to heat the cofferdam 10 so as to allow aworker to enter the internal space of the cofferdam 10, and the heatinsulating material 120 provided in the cofferdam 10.

The present embodiment has a difference from the foregoing first andsecond embodiments in that the floating structure includes the gassupplier 320 supplying gas into the cofferdam 10 to easily find out acold spot formed in the bulk head 11 of the cofferdam 10. The cofferdam10, the heater 30, and the heat insulating material 120 described in theforegoing first and second embodiments may be applied to the presentembodiment as they are.

In the floating structure according to the present embodiment, a workeralso periodically enters the cofferdam 10 to inspect whether there is acold spot in the bulk head 11 of the cofferdam 10. That is, it needs tobe inspected whether a cold part occurs at a specific part of the bulkhead 11 of the cofferdam 10. From this, it may be appreciated that thebulk head 11 is covered with frost and a visual inspection is performed.

However, when the temperature of the cofferdam 10 is maintained to belower at temperature below 0° C. and the inside of the cofferdam 10 isfilled with general air, the whole bulk head 11 of the cofferdam 10 iscovered with frost and therefore the cold spot may not be found based onpresence and absence of the frost.

According to the present embodiment, the cofferdam 10 is filled withgas, for example, the dry air and the temperature of the bulk head 11 ofthe cofferdam 10 is controlled to be higher than a dew point of the dryair to cover only the bulk head 11 having temperature lower than the dewpoint temperature of the dry air with frost, thereby easily finding outthe cold spot.

For example, when the dew point temperature of the dry air generated inthe LNG carrier is −40° C., the temperature of the bulk head 11 of thecofferdam 10 is controlled to be −35° C. and if a worker enters thecofferdam 10 to perform a visual inspection, the bulk head 11 of thecofferdam 10 having a temperature lower than −40° C. is covered withfrost, and therefore the cold spot may be easily found based on theposition of the frost.

Further, a technology means for supplying the dry air having the lowerdew point temperature into the foregoing cofferdam 10 may be applied toa trunk deck space TS (see FIG. 21) and a side passage way SP (see FIG.21) contacting a trunk deck TD as it is in a sixth embodiment of thepresent invention to be described below.

Further, when the temperature of the bulk head 11 of the cofferdam 10 iscontrolled to be −35° C., as shown in a Table of FIG. 18, the BOR may bedecreased by about 4.9% compared to the case of controlling thetemperature of the bulk head 11 to be 5° C. In this case, the bulk head11 may be made of the low temperature steel LT.

According to the present embodiment, as illustrated in FIG. 17, the gassupplier 320 includes a gas supply pipe 321 provided in the cofferdam 10to supply gas supplied through a gas supply line AL into the cofferdam10, a gas discharge pipe 322 provided in the cofferdam 10 to dischargethe internal gas of the cofferdam 10 to the outside of the cofferdam 10,and opening and closing valves 323 provided in the gas supply pipe 321and the gas discharge pipe 322.

According to the present embodiment, the dry air supplied to the gassupply line AL may be supplied from a dry air generator installed in theexisting LNG carrier and therefore additional costs for the facility donot occur.

According to the present embodiment, the dry air supplied to thecofferdam 10 may have the dew point temperature from −45° C. to −35° C.and the temperature of the bulk head 11 of the cofferdam 10 may becontrolled to be 1° C. to −10° C. higher than the dew point temperatureof the dry air. In this case, the temperature of the bulk head 11 iscontrolled to be about −30° C., and therefore the BOR may be decreased.

For the reason of inspection, maintenance, etc., when a worker needs toenter the cofferdam 10, he/she may put on winter clothes to bear lowtemperature, thereby performing the work. On one hand, the foregoing gasis continuously injected into and vented from the cofferdam 10 toincrease the temperature of the cofferdam, and as a result, a worker mayenter the cofferdam to perform the work.

FIG. 19 is a diagram schematically illustrating controlling thetemperature of the cofferdam depending on a change in pressure of an LNGstorage tank in a floating structure according to a fifth embodiment ofthe present invention.

A heat insulating system 400 of the floating structure according to thepresent embodiment includes the cofferdam 10 that is provided betweenthe plurality of LNG storage tanks T to dispose the plurality of LNGstorage tanks T in multi rows in at least one direction of the lengthdirection and the width direction of the hull and is controlled to betemperature below 0° C. and the heater 30 that is provided in thecofferdam 10 to heat the cofferdam 10, in which the present embodimenthas a difference from the foregoing first embodiment in that thecofferdam 10 is heated by the heater 30 to control the sub-zerotemperature of the cofferdam 10 to be temperature above 0° C. and thetemperature of the cofferdam 10 is controlled depending on the change inpressure in the LNG storage tank T and the rest contents of the firstembodiment may be applied to the present embodiment as they are.

That is, according to the present embodiment, when the temperature ofthe cofferdam 10 is maintained at temperature below 0° C. to lower theBOR and the BOG is little generated according to the sailing conditions,and thus more BOG is required for the reason of ship fuel, etc., thetemperature of the cofferdam 10 is increased to make the BOR larger andgenerate more BOG and when too much BOG is generated according to thesailing conditions and thus it is difficult to treat the BOG, thetemperature of the cofferdam 10 is lowered to make the BOR smaller andless generate the BOG.

The foregoing control temperature may be manually set in considerationof the sailing conditions, etc., and may also be automatically set byreceiving a pressure signal of the LNG storage tank T. That is, when thepressure of the LNG storage tank T is high, the BOG is excessivelygenerated and therefore the set value of the control temperature may becontrolled to be low and when the pressure is low, the BOG is lessgenerated and therefore the set value of the control temperature may becontrolled to be high.

Further, the present embodiment has a difference from the foregoingfirst embodiment in that the temperature of the cofferdam 10 ismaintained at temperature below 0° C. to decrease the BOR and thetemperature of the cofferdam 10 may be controlled to be a specifictemperature (for example, temperature above 0° C.) so that a worker mayenter the cofferdam 10.

In detail, to inspect whether the cold spot, etc., occurs in thecofferdam 10 during the sailing, a worker needs not enter the cofferdam10.

Even in this case, if the cofferdam 10 is maintained at temperaturebelow 0° C., a worker entering the cofferdam 10 to perform the work isexposed to a low temperature and thus may be in danger. Therefore, theset value of the control temperature is increased and thus the heater 30heats the cofferdam 10 to maintain the cofferdam 10 at the specifictemperature (for example, temperature above 0° C.)

According to the present embodiment, when the bulk head 11 of thecofferdam 10 is made of a material bearing a temperature from −30° C. to0° C., the temperature of the cofferdam 10 may be controlled to be in arange from −30° C. to 70° C. For example, when a worker need not toenter the cofferdam 10, to maximally decrease the BOR, the controltemperature of the cofferdam 10 may be controlled to be about −30° C. Onthe other hand, the cofferdam 10 may be controlled to be a specifictemperature in addition to temperature above 0° C.

According to the present embodiment, when the bulk head 11 of thecofferdam 10 is made of the low temperature steel LT bearing up to atemperature of −55° C., the temperature of the cofferdam 10 may becontrolled to be in a range from −55° C. to 70° C. For example, when aworker need not to enter the cofferdam 10, to maximally decrease theBOR, the temperature of the cofferdam 10 may be controlled to be about−50° C. On the contrary, the cofferdam 10 may be controlled to be aspecific temperature in addition to temperature above 0° C.

Hereinafter, to permit a worker to enter the cofferdam 10, a method forcontrolling a temperature of a cofferdam will be described.

First, the temperature of the cofferdam 10 is controlled to temperaturebelow 0° C., for example, −25° C. or −50° C. to decrease the heattransfer between the cofferdam 10 and the LNG stored in the LNG storagetank T, and therefore if a worker immediately enters the cofferdam,he/she may be in danger.

Therefore, a step of heating the cofferdam 10 at temperature above 0° C.by the heater 30 is performed. In this case, the cofferdam 10 may beheated with a glycol heating coil 31, an electrical coil, and a coil inwhich steam or clear water flows or heated by supplying high temperatureair into the cofferdam 30.

Next, if the temperature in the cofferdam 10 becomes temperature above0° C., a worker enters the cofferdam 10 to confirm whether the coldspot, etc., occurs in the bulk head 11. In this case, the inside of thecofferdam 10 is continuously maintained at temperature above 0° C.

If a worker completes the internal inspection of the cofferdam 10 to getout of the cofferdam 10, the heating of the cofferdam 10 stops to againmaintain the cofferdam 10 at temperature below 0° C.

As described above, according to the present embodiment, when a workerdoes not enter the cofferdam 10, the cofferdam 10 may be maintained attemperature below 0° C. to decrease the BOR. On the other hand, thecofferdam 10 may be maintained at temperature above 0° C. to permit aworker to perform the work and consider safety of a worker whiledecreasing the BOR.

Further, the foregoing technology means for controlling a temperature ofa cofferdam 10 may also be applied to the trunk deck space TS (see FIG.21) and the side passage way SP (see FIG. 21) contacting the trunk deckTD as it is in the sixth embodiment of the present invention to bedescribed above.

Further, the present embodiment has a difference from the foregoingfirst embodiment in that the temperature of the cofferdam 10 may becontrolled depending on the change in pressure in the LNG storage tankT.

In detail, according to the present embodiment, as illustrated in FIG.19, a pressure sensor PT that may measure the pressure in the LNGstorage tank T is provided in the LNG storage tank T and then thetemperature of the cofferdam 10 may be controlled based on the pressuremeasured by the pressure sensor PT.

That is, if the pressure of the LNG storage tank T is increased, the BOGis more generated than the BOG required in the floating structure andtherefore the setting temperature of the temperature control of thecofferdam 10 is lowered to lower the temperature of the cofferdam 10,thereby decreasing the BOG. If the pressure of the LNG storage tank T isdecreased, the BOG is less generated than the BOG required in thefloating structure and therefore the setting temperature of thetemperature control of the cofferdam 10 is increased to increase thetemperature of the cofferdam 10, thereby generating more BOG.

Further, the temperature of the cofferdam 10 may also be controlled byreferring to a velocity of the floating structure independent of thepressure sensor PT.

In detail, when the velocity of the floating structure is increased toincrease the fuel consumption, the control temperature of the cofferdam10 may be increased to more generate BOG and the generated BOG may beused as fuel to fit the fuel consumption.

For example, in the floating structure controlling the bulk head 11 ofthe cofferdam 10 to be the setting temperature of −25° C., the BORbecomes 0.1236. When the velocity of the floating structure is increasedto consume more fuel, if the temperature of the bulk head 11 of thecofferdam 10 is controlled to be 0° C., the BOR becomes 0.1282 and thusis increased by 3.7%, thereby increasing the BOG. Therefore, when thevelocity of the floating structure is increased and thus the consumptionof the BOG is increased, the insufficient amount of BOG may bedecreased.

On the contrary, when the velocity of the floating structure isdecreased and thus the fuel consumption is decreased, the controltemperature of the cofferdam 10 is lowered to less generate the BOG,thereby fitting the fuel consumption.

Meanwhile, when the bulk head 11 of the cofferdam 10 is heated by theheater 30, the heat transfer is made by conduction and thus the heatingtime of the cofferdam 10 may be required, such that the hot dry air maybe supplied into the cofferdam 10 to shorten the heating time of thecofferdam 10.

In addition, the gas supplier and the gas supplier 320 described in theforegoing embodiments may be applied to the present embodiment as theyare.

FIG. 20 is a diagram schematically illustrating a state in which a heatinsulating material is provided in a trunk deck space TS and a sidepassage way in a heat insulating system of a floating structureaccording to a sixth embodiment of the present invention and FIG. 21 isa table showing calculated results of a BOR generated by controlling atemperature of an internal hull IH contacting the trunk deck space TSand the side passage way illustrated in FIG. 20.

A heat insulating system 500 of the floating structure according to thepresent embodiment includes the heat insulating material 120 provided inat least one of the trunk deck space TS and the side passage way SPcontacting the trunk deck TD to decrease the heat transfer from the deckspace TS or the side passage way SP into the plurality of LNG storagetank T, thereby decreasing the BOR generated due to the heat transfer.

According to the present embodiment, the temperature of the internalhull IH contacting the trunk deck space TS and the side passage way SPmay be lowered to decrease the heat invasion from the outside, therebydecreasing the BOR.

In particular, in the case of sailing the floating structure along aroute having very low ambient temperature like a north pole route orsailing the floating structure during the winter, if the presentembodiment is applied, the internal hull IH contacting the trunk deckspace TS and the side passage way SP may be lowered to decrease the BOR.

On the contrary, even in the case of sailing the floating structure at aplace where temperature is high or sailing the floating structure duringthe summer, the temperature of the internal hull IH contacting the trunkdeck space TS and the side passage way SP is lowered by the heatinsulating material 120 to maintain the temperature of the cofferdam 10at low temperature, thereby decreasing the BOR.

In particular, the trunk deck TD and the side passage way SP contactingthe trunk deck TD are directly exposed by solar heat and therefore ifthe heat insulating material 210 is provided here, the heat invasionfrom the outside may be decreased and thus the BOR may be moreeffectively decreased.

As a result of calculating the BOR for the actual LNG carrier based onnumerical analysis, as illustrated in a table in FIG. 21, when thetemperature of the internal hull IH contacting the trunk deck space TSand the side passage way SP is not controlled, the temperature of theinternal hull IH becomes about 35.3° C. In this case, the BOR iscalculated as 0.1346.

However, when the present embodiment is applied to control thetemperature of the internal hull IH contacting the trunk deck space TSand the side passage way SP to be 0° C., as shown in a table of FIG. 21,it may be appreciated that the BOR becomes 0.1296 and thus is decreasedby about 3.7%. The BOR may be decreased using the cheap heat insulatingmaterial 120 and it may be appreciated that the great BOR decreaseeffect compared to price may be obtained.

As another example, when the present embodiment is applied to controlthe temperature of the internal hull IH contacting the trunk deck spaceTS and the side passage way SP to be −25° C., it may be appreciated thatthe BOR becomes 0.1266 and thus is decreased by about 5.9%. Likewise, itmay be appreciated that upon the use of the heat insulating material,the great BOR decrease effect compared to price may be obtained.

As illustrated in FIG. 20, the heat insulating material 120 may beprovided at an inside ceiling part of the trunk deck TD, a ceiling partand a side wall part of the side passage way SP of the trunk deck TD,and a part of the side passage way SP contacting a ballast tank BT.

According to the present invention, the heat insulating material 20 isnot provided at the position of the trunk deck TD but may be provided atthe bottom part or the outside part of the trunk deck TD and may also bediscontinuously or continuously provided in the trunk deck space TS andthe side passage way SP.

Further, the present embodiment may use the heat insulating material 120of the foregoing embodiment as it is. That is, the heat insulatingmaterial 120 of the present embodiment may be a heat insulating wall ofthe sealing and heat insulating unit SI sealing and heat insulating theLNG storage tank T and may include at least one of the panel type heatinsulating material, the foam type heat insulating material, the vacuumheat insulation or particle type heat insulating material, and thenon-woven fabrics type heat insulating material. Further, the presentinvention does not limit a kind, shape, and installation method of theheat insulating material.

The present embodiment may include the heater 30 heating the internalhull IH to heat the cofferdam 10 or maintain the internal hull IH as thedesired temperature. The configuration of the heater 30 may include theglycol heating coil 31, the electrical coil, the coil in which a liquidsuch as the steam and the clear water flows, or the like of theforegoing embodiment.

According to the present embodiment, the control of the material and thetemperature of the internal hull IH contacting the trunk deck space TSand the side passage way SP may be selectively made depending on therequired value of the BOR.

In detail, according to the present embodiment, the internal hull IH maybe controlled to be −55° C. to 30° C. Preferably, to use the material ofthe internal hull IH as the steel grade A defined in the IGC, theinternal hull IH may be controlled to be 0° C. to 30° C. For example, ifthe temperature of the internal hull IH is controlled to be 0° C., asshown in a table of FIG. 21, compared to the existing embodimentcontrolling the temperature of the internal hull IH to be 35.3° C., theBOR may be 0.1296 decreased by 3.7% and the internal steel IH may alsouse the steel grade A.

Further, if the temperature of the internal hull IH is controlled to be−25° C., as shown in a table of FIG. 21, the BOR may be 0.1266 decreasedby 5.9% and the internal steel IH may also use the steel grade E or EH.In addition, if the temperature of the internal hull IH is controlled tobe −30° C. or less, the internal hull IH may be made of the lowtemperature steel LT.

Meanwhile, the contents of the cofferdam 10, the gas supplier 320, andthe gas supplier of the foregoing embodiment may be applied to thepresent embodiment as they are.

FIG. 22 is a diagram schematically illustrating a state in which a heatinsulating material is provided in a ballast tank in a heat insulatingsystem of a floating structure according to a seventh embodiment of thepresent invention and FIG. 23 is a table illustrating calculated resultsof a BOR generated by controlling a temperature contacting an internalhull contacting the ballast tank.

A heat insulating system 600 of the floating structure according to thepresent embodiment includes the heat insulating material 120 provided inthe ballast tank BT to decrease the heat transfer from the ballast tankBT into the LNG storage tank T, thereby decreasing the BOR.

According to the present embodiment, the temperature of the internalhull IH contacting the LNG storage tank T in the ballast tank BT may belowered to decrease the heat invasion from the outside, therebydecreasing the BOR.

Even when the floating structure is sailed toward a place wheretemperature is high or sailed during the summer, the temperature of theinternal hull IH contacting the LNG storage tank T in the ballast tankBT may be lowered by the heat insulating material 120 to decrease theBOR.

In detail, according to the present embodiment, the temperature of theinternal hull IH contacting the ballast tank BT and the LNG storage tankmay be controlled to be −55° C. to 30° C. Preferably, to use thematerial of the internal hull IH as the steel grade A defined in theIGC, the internal hull IH may be controlled to be a temperature of 0° C.to 20° C.

As a result of calculating the BOR with the numerical analysis for theactual LNG carrier, when the temperature of the internal hull IHcontacting the ballast tank BT and the LNG storage tank are notcontrolled, as shown in a table of FIG. 23, the temperature of theportion is about 27.2 to 36. 13° C. In this case, the BOR is calculatedas 0.1346.

However, when the present embodiment is applied to control thetemperature of the internal hull IH contacting the LNG storage tank T inthe ballast tank BT to be 0° C., as shown in a table of FIG. 23, it maybe confirmed that the BOR becomes 0.1242 and thus is decreased by about7.7%. That is, the BOR may be decreased using the cheap heat insulatingmaterial 120, and therefore it may be appreciated that the great BORdecrease effect compared to price may be obtained.

Further, as another example, even when the temperature of the internalhull IH contacting the ballast tank BT and the LNG storage tank iscontrolled to be 5° C., it may be confirmed that the BOR becomes 0.1262and thus is decreased by about 6.2%. Likewise, it may be appreciatedthat upon the use of the heat insulating material, the great BORdecrease effect compared to price may be obtained.

As illustrated in FIG. 22, the heat insulating material 120 may beprovide at a ceiling wall of the ballast tank BT of the region in whichthe inside of the external hull EN contacts the ballast tank BT and theside passage way.

Further, the present embodiment may use the heat insulating material 120of the foregoing embodiment as it is. That is, the heat insulatingmaterial 120 of the present embodiment may be a heat insulating wall ofthe sealing and heat insulating unit SI sealing and heat insulating theLNG storage tank T and may include at least one of the panel type heatinsulating material, the foam type heat insulating material, the vacuumheat insulation or particle type heat insulating material, and thenon-woven fabrics type heat insulating material. Further, the presentinvention does not limit a kind, shape, and installation method of theheat insulating material.

The present embodiment may include the heater 30 heating the internalhull IH to heat the cofferdam 10 or maintain the internal hull IHcontacting the ballast tank BT at the desired temperature. Theconfiguration of the heater 30 may include the glycol heating coil 31,the electrical coil, the fluid coil in which a liquid such as the steamand the clear water flows, or the like of the foregoing embodiment.

According to the present embodiment, the control of the material and thetemperature of the internal hull IH contacting the ballast tank BT maybe selectively made depending on the required value of the BOR.

In detail, according to the present embodiment, the internal hull IHcontacting the ballast tank BT may be controlled to be a temperature of−55° C. to 30° C. If the temperature of the internal hull IH iscontrolled to be 0° C., as shown in the table of FIG. 23, compared tothe existing embodiment controlling the temperature of the internal hullIH to be 27.1° C. to 36.1° C., the BOR may be 0.1242 decreased by 7.7%and the internal steel IH may also use the steel grade A.

Further, if the temperature of the internal hull IH is controlled to be5° C., as shown in the table of FIG. 23, the BOR may be 0.1262 decreasedby 6.2% and the internal steel IH may also use the steel grade A.

Meanwhile, the contents of the cofferdam 10 and the gas supplier 320 ofthe foregoing embodiment may be applied to the present embodiment asthey are. However, the gas supplier 320 may not be applied in the statein which the ballast tank BT is filled with the number of ballasts andtherefore may be applied only to the cofferdam 10.

Although the embodiments of the present invention have been disclosedfor illustrative purposes, those skilled in the art will appreciate thatvarious modifications, additions and substitutions are possible, withoutdeparting from the scope and spirit of the invention as disclosed in theaccompanying claims. Accordingly, such modifications, additions andsubstitutions should also be understood to fall within the scope of thepresent invention.

1. A heat insulating system of a floating structure, wherein a heatinsulating material is provided at at least one of a ballast tank, atrunk deck space, and a side passage way contacting a trunk deck todecrease a boil-off rate (BOR) generated due to a heat transfer from anexternal region of a hull into an LNG storage tank.
 2. The heatinsulating system of a floating structure of claim 1, wherein atemperature of an internal hull contacting at least one of the ballasttank, the trunk deck space, and the side passage way contacting thetrunk deck is controlled to be −55 to 30° C.
 3. The heat insulatingsystem of a floating structure of claim 2, wherein the temperature ofthe internal hull contacting the ballast tank is controlled to be 0 to20° C. and the internal hull is made of steel grade A defined in IGC. 4.The heat insulating system of a floating structure of claim 2, whereinthe temperature of the internal hull contacting at least any one of thetrunk deck space and the side passage way contacting the trunk deck iscontrolled to be 0 to 30° C. and the internal hull is made of the steelgrade A defined in the IGC.
 5. The heat insulating system of a floatingstructure of claim 2, wherein the temperature of the internal hullcontacting at least any one of the trunk deck space and the side passageway contacting the trunk deck is controlled to be −30° C. or less andthe internal hull is made of low temperature steel LT.
 6. The heatinsulating system of a floating structure of claim 1, wherein the heatinsulating material is provided on an inside wall of an external hullforming the ballast tank.
 7. The heat insulating system of a floatingstructure of claim 1, wherein the heat insulating material is providedin a region in which the side passage way close to the trunk deck spacecontacts the ballast tank.
 8. The heat insulating system of a floatingstructure of claim 1, wherein the heat insulating material includes atleast one of the heat insulating wall heat-insulating LNG stored in theLNG storage tank, a panel type heat insulating material, a foam typeheat insulating material, a vacuum type heat insulating material, aparticle type heat insulating material, and a non-woven fabrics typeheat insulating material.
 9. The heat insulating system of a floatingstructure of claim 2, further comprising: a heater heating the internalhull.
 10. The heat insulating system of a floating structure of claim 7,wherein the heater uses at least of a glycol heating coil, an electricheating coil, and a fluid pipe to heat the internal hull.
 11. The heatinsulating system of a floating structure of claim 1, wherein thefloating structure is any one selected from an LNG FPSO, an LNG FSRU, anLNG carrier, and an LNG RV.
 12. A heat insulating system of a floatingstructure, comprising: a cofferdam provided between a plurality of LNGstorage tanks installed in at least one row in a length direction of ahull; and a heat insulating material provided in a ballast tank, a trunkdeck space, and a side passage way contacting a trunk deck, wherein thecofferdam is controlled to be a temperature of 0° C. or less to decreasea heat transfer from the cofferdam into the plurality of LNG storagetanks and controls a temperature of at least one of the ballast tank,the trunk deck space, and the side passage way contacting the trunk deckto be lower than a temperature of an external hull to decrease aboil-off rate (BOR) generated due to the heat transfer from at least oneof the ballast tank, the trunk deck space, and the side passage waycontacting the trunk deck into the plurality of LNG storage tanks. 13.The heat insulating system of a floating structure of claim 12, whereinthe cofferdam includes: a pair of bulk heads spaced apart from eachother between the plurality of LNG storage tanks; and a space portionprovided by the pair of bulk heads and an inner wall of the hull, andthe cofferdam controls the pair of bulk heads to be temperature below 0°C.
 14. A heat insulating system of a floating structure, wherein atemperature of an internal hull contacting at least one of a ballasttank, a trunk deck space, and the side passage way contacting a trunkdeck is controlled to be −55 to 30° C.